1. Field of the Invention
The present invention relates to a protective body to keep a fixing element on the distal end of an implantable lead from coming into contact with and damaging the walls of a body cavity during introduction of the lead into the cavity, the protective body being of the type formed by a covering of a material which, following introduction and contact with body fluid, and when pressure is applied to the material against body tissue, allows mechanical penetration of the protective body by the fixing element enabling the fixing element to be anchored in the body cavity.
2. Description of the Prior Art
An implantable lead, i.e. an electrode cable, intended for connection at one end, the proximal end, to a medical device, such as a pacemaker, and whose other end, the distal end, is to be affixed in a ventricle or atrium of the heart, can be passively or actively affixed to the heart wall, after intravenous introduction into the heart, in order to transmit electrical pulses to heart muscle, thereby inducing this tissue to contract, or to sense the heart's intrinsic activity in control of the pacemaker's operating cycle. For passive fixing of the distal end of the lead, this lead end is usually equipped with small tines or barbs to facilitate anchoring in the heart wall without penetrating the heart wall.
In order to achieve rapid, reliable fixing of the distal end of the lead to the heart wall, e.g. when the end of the lead is to have a J shape and press against an atrial wall, active fixing of the end to the atrial wall is preferred. For this purpose, the end of the lead has a fixing element which can be driven into wall tissue in implantation, a process which can be monitored by fluoroscopy to ensure that the end of the lead is correctly positioned.
A commonly used fixing element for this purpose has the shape of a pointed, helical screw. Such lead ends are screw-in types and require active insertion of the fixing element into the heart wall. In cases where the helical screw can freely rotate on the end of the lead, the helical screw can be rotated with the aid of a stylet which is inserted from the proximal end of the electrode cable and made to grip a polygonal recess in the proximal end of the helical screw, the screw then being screwed into and affixed to the heart wall with the end of the electrode pressing against the wall.
In order to keep the pointed tip of the helical screw from coming into contact with and damaging venous and heart walls during the implantation of the lead, the aforementioned freely rotating helical screw can initially be retracted inside a protective recess in the end of the lead and can be deployed by the stylet only after it arrives at the intended implantation site, just before the helical screw is screwed in.
Another type of fixing element for active fixing of the end of the lead to the heart wall has, on the distal end of the lead, a non-rotating helical screw which projects from that end. In this instance, the helical screw must be provided with some kind of protection to keep the tip of the helical screw from damaging venous and heart walls during the implantation of the lead. The entire lead, including its covering, must then be rotated in order to screw the helical screw into the heart wall.
A number of different solutions have been proposed for preventing such projecting, non-rotating helical screw from causing damage during introduction. One proposal is described in U.S. Pat. No. 3,974,834 in which a compressible, bellows-like sleeve on the distal end of the lead protects the helical screw during intravenous introduction of the lead, an dis compressed when pressed against the heart wall, so the tip of the helical screw can get a foothold before being screwed into the heart wall.
The solution according to U.S. Pat. No. 3,974,834 has some disadvantages. A relatively large amount of foreign material is introduced into the heart, thereby imposing a load on the heart wall. In some patients, this can disrupt heart activity. In addition, the bellows-like sleeve is not mechanically locked during intravenous introduction This means that the helical screw could be exposed prematurely and inflict damage on vessels and the heart during careless introduction.
U.S. Pat. Nos. 4,827,940 and 4,876,109 disclose relatively hard bodies made of a water-soluble substance, such as mannitol, which covers the helical screw and which dissolves in body fluids within a few minutes, whereupon the helical can be screwed into the heart wall. As long as a body according to any of these documents remains in place, the helical screw covered by the body cannot damage vessels or the heart.
The protective bodies described in U.S. Pat. Nos. 4,827,940 and 4,876,109 dissolve rather rapidly in blood, and the helical screw means is exposed in 2-4 minutes. Experiences physicians might regard this dissolution time as long, and their inability to govern this time themselves is unsatisfactory to many. Inexperienced physicians, on the other hand, may view this dissolution time as brief, causing them to feel stressed.
U.S. Pat. Nos. 4,106,512 and 4,311,153 each disclosed leads equipped with protective bodies made of silicon rubber. In contrast to the protective bodies of the previously cited patents, the silicon rubber bodies are insoluble in body fluids. Hence, the silicon rubber bodies will remain in the heart after the lead has been affixed to the heart wall, and it is just as easy for the helical screw to penetrate the protective body when the lead enters the body as it is just before the lead is to be affixed to the heart.
Consequently there is a need for a new type of small, simple protective body for covering the helical screw on a pacemaker lead with the aid of which controlled exposure of the electrode's helical screw is possible immediately before the screw is screwed into the heart wall.